The ability of eukaryotic cells to proliferate in response to a growth signal is tightly controlled by a complex network of ordered biochemical events collectively known as the cell cycle. Mitogenic signals commit cells to entry into a series of regulated steps of the cell cycle. The synthesis of DNA (S phase), and separation of two daughter cells (M phase) are the main features of cell cycle progression. The G1 phase separates the M and S phases and prepares the cell for DNA duplication upon receiving mitogenic signals. The period between the S and M phases is known as the G2 phase during which cells repair errors that occurred during DNA duplication.
Regulators of the cell cycle have gained widespread importance in proliferative diseases. Cyclin-dependent kinases (CDKs) are a family of enzymes which become activated in specific phases of the cell cycle. CDKs consist of a catalytic subunit (the actual cyclin-dependent kinase or CDK) and a regulatory subunit (cyclin). There are at least nine CDKs (CDK1, CDK2, CDK3, CDK4, CDK5, CDK6, CDK7, CDK8, CDK9, etc.) and 15 different types of cyclins (cyclin A, B1, B2, D1, D2, D3, E etc). Each step of the cell cycle is thought to be regulated by such CDK complexes: G1/S transition (CDK2/cyclin A, CDK4/cyclin D1-D3, CDK6/cyclin D3) (Senderwicz A. M. and Sausville E. A., J. Natl. Cancer Inst. 2000, 376-387), S phase (CDK2/cyclin A), G2 phase (CDK1/cyclin A), G2/M transition phase (CDK1/cyclins B).
CDKs are able to phosphorylate many proteins that are involved in cell cycle events, including tumor suppressor proteins, such as the retinoblastoma gene product Rb. The Rb is involved in the G1/S transition of the cell cycle and its phosphorylation by CDKs results in its inactivation (U.S. Pat. No. 5,723,313), which in turn leads to the release of the transcriptional factor E2F and the activation of E2F-responsive genes necessary for progression to the S phase.
A wide variety of diseases are characterized by uncontrolled cell proliferation that results from some fault in the regulatory pathways in the cell cycle [e.g. overexpression of cyclins or deletions of genes encoding CKIs (CDK inhibitory proteins)]. The overexpression of cyclinD1 leads to the deregulation of CDK4-D1 kinase activity and thereby contributes to uncontrolled cell proliferation. With knowledge of the role of CDKs in cell cycle regulation and the discovery that approximately 90% of all neoplasias are associated with CDK hyperactivation leading to the inactivation of the Rb pathway, CDKs are attractive targets for the development of anti-tumor drugs.
The first potent molecule to be developed as an effective CDK inhibitor was a flavone compound, namely flavopiridol [cis-{2-(2-Chloro-phenyl)-5,7-dihydroxy-8-(3-hydroxy-1-methyl-piperidin-4-yl)-chromen-4-one hydrochloride}]. Flavopiridol is known to inhibit different CDKs and it exhibits anti-proliferative activity in vitro against a broad range of human cancer cells. Further research on flavones as a class of compounds offers a potential approach to anti-proliferative therapy. As a result, analogs of flavopiridol have been the subject of other publications. U.S. Pat. No. 5,733,920 describes novel chromone analogs as inhibitors of CDK/Cyclin complexes. U.S. Pat. No. 5,849,733 discloses 2-thio and 2-oxo analogs of flavopiridol as protein kinase inhibitors for the treatment of proliferative diseases. WO 01/83469 discloses 3-hydroxychromen-4-one derivatives as inhibitors of cyclin dependent kinases. U.S. Pat. No. 5,116,954 and US H1427 disclose flavonoid compounds having anticancer and immunomodulatory activity. U.S. Pat. No. 5,284,856 discloses use of benzopyran-4-one derivatives for the control of tumoral diseases. U.S. Pat. No. 4,900,727 discloses benzopyran-4-one derivatives antiinflammatory agents. Anti-inflammatory benzopyran-4-one derivative from Dysoxylum binectariferum is described by R. G. Naik et al in Tetrahedron, 1988, 44 (7), 2081-2086.
The prominent role of CDK/cyclin kinase complexes, in particular CDK4/cyclin D kinase complexes, in the induction of cell proliferation and their deregulation in tumors, makes them ideal targets for developing highly specific anti-proliferative agents.
There is a clear need, however, for CDK inhibitors which can be used as anti-proliferative agents in an efficient or more specific manner. A focused research on CDK inhibitors by the present inventors resulted in the discovery of novel flavone analogs possessing structural features not envisaged in the prior art, as effective inhibitors of CDKs. Moreover, the compounds of the invention inhibit CDKs effectively with greater selectivity than the known CDK inhibitors, which are under clinical trials (Curr. Pharm. Biotechnol. 2000, July (1): 107-116) and also show comparatively low cytotoxicity against various different proliferative cell lines. Therefore, the compounds of the present invention are candidate agents for the treatment of various cell proliferation related disorders.